There are numerous types of shirts. Three of the most common types are shirts that button in the front of the wearer, those that button in the back of the wearer, and those that are pulled over the head of the wearer. These shirts may have various kinds of collars, cuffs, lengths, and sleeve lengths. They are made of a variety of materials. They may be made of various types of cloth, leather, plastic, or other materials. Some of the principal purposes of a person wearing a shirt are to provide warmth, to protect one from the wind, to protect one from the sunlight, to provide a fashionable cover for the upper part of the body, or just to cover the top part of the body so it isn't bare.
There are many times that people don't wear shirts. During these times the upper part of the body is exposed to the elements such as the wind, rain, or sunlight.
People often don't wear shirts when the temperature around them is hot. Some of these times are when these people are outdoors. During these times the unshirted upper part of the body would be exposed to sunlight. Some types of people who might not wear a shirt outside in hot weather and might stay in the sunlight for considerably long lengths of time are construction workers, farmers, and fishermen. Other shirtless people who might stay in the sunlight for considerably long periods of time are those engaged in games such as tennis or basketball.
There is now medical evidence that exposure of the skin to the sunlight for long periods of time causes skin to age prematurely and causes skin cancer.
The effect of sunlight on the skin of the human body is becoming increasingly more important as the atmospheric shield of the earth is becoming less effective in screening out harmful rays of light or radiation and such light or radiation reaches the surface of the earth and the people inhabiting it.
The reason the people mentioned above don't wear shirts is because wearing a shirt would cause their bodies to be hotter than they are without a shirt. If they wore a shirt they might get hot, sweat, and be uncomfortable. In the past, many of them have probably not even considered that they were exposing their bodies to the harmful rays of the sun. In fact, many of them probably wanted to expose their bodies to the sunlight thinking that it was good for them, or they may have wanted to get their bodies to have a tan color.
The people who believed too much sunlight was a danger might wear a shirt if they had a shirt that protected them from the sunlight and did not cause their bodies to get excessively hotter. My invention is such a shirt.
The sunlight does not strike all parts of the upper body for an equal time or in the same manner. The back and sides of the neck, the shoulders, the outside or back portions of the arms, the back, and the waist are exposed to the direct light of the sun more of the time than is the rest of the upper body. This is because the people mentioned above usually have their upper body bent forward when they are engaged in the activities mentioned. The sunlight is striking their chests, the inside of their arms, and the front of their necks less of the time, and it is hitting these parts indirectly (reflected light) or it is hitting these parts at angles which lessen the harmful effect of the rays.